Method of bleaching groundwood



,JU steal: za-ziuaauuvls Misti-12bit ii iii-JUNE P198309 2,510,595 Patented June 6, 1950 METHOD OF BLEACHING GROUNDWOOD I Robert L. McEwen, Williamsville, and Fred R.

Sheldon, Buffalo, N. Y.,' assignors to Buffalo Electro-Chemical Company, Inc., Tonawanda, N. Y;

No Drawing.

The present invention relates to a method of bleaching or brightening a web or sheet of pulp or paper containing substantial proportions of ground'wood fibers and therefore asubstantial amount of natural wood incrustants or ligneous matter, and particularly to the bleaching of such webs or sheets by the employment of an alkaline solution containing. a-peroxide and a phosphate. So-called chemical pulps, whether made by the soda, sulfate or sulfite methods, have been treated chemically to obtain relatively pure cellulose fibers from the original w'o'odby the elimination of the non-cellulosic constituents designated incrustants and lignins. 'These substantially pure cellulose fibers, freed of the incrustants and lignins, may be readily bleached by hypochlorite In contrast to the chemical pulps there are many types of pulp mixtures which contain all or a portion of the incrustations, lignins and other non-cellulosic. materials. Groundwood is the best known of this classification. Groundwood pulp and other pulps falling under this classification are herein described as pulps containing groundwood and the fibers of these pulps contain lignin in amounts greater than about 5-6% by weight. 5

Groundwood fibers 'contain'all the natural incrustants and lignins of the original wood. Groundwood fibers therefore are not of such good initial white and furthermore are difficult to bleach by reasonof the presence of the ligneous or other non-cellulosic matter of the original wood. Whereas it has been proposed to bleach chemical pulps,'that is, those pulps substantially free of the natural incrustants and lignins, with alkaline peroxide solutions containing caustic sodaand sodium' silicate,-' as for instance by the methods described in Kaufimann and Wolfe U. S. Patent No. 2,150,926 and Heritage U. S. Patent No. 2,125,634, such operations possess little or no value in the treatment of pulps containing groundwood. Relatively good bleaching results Application September 20, 1946, Serial No. 698,402

2 Claims. (01. 8-104) wood pulp with bleaching solutions of peroxide and silicate in the. manner employed upon chemi-.-

amounts by the employment of an alkaline peroxide solution.

It istherefore an objectof the invention to obtain a relatively high degree of brightening efiiciency upon webs containing relatively high proportions of ligneousmatter or considerable ground wood pulp.

It is also an object of the invention to perform a brightening of webs containing ligneous matter on paper machines of conventional construction.

vIn accordance with the present invention, webs of pulp or paper sheets containing ground wood fibers, that is, matted or felted sheets of vegetable fiber which have been formed on a screen from a water suspension, are treated with an alkaline peroxide solution containing a phosphate and the sheet 'orweb permitted to bleach or brighten by the subsequent conjoint action of the peroxide and phosphate. The solution may be applied to one or both sides of the web or sheet and in sufficient quantities to produce the degree of bleaching desired. The sheet after application of the aqueous solution of peroxide containing a phosphate should contain from about 12% to 250% of water based on the dry weight of the pulp whereby the sheet is still selfsustaining while the alkaline aqueous solution of Y the peroxideand phosphate should possess an initial pH of from 9.5 to 11.5. The phosphates employed should be such that the solutions fall within the alkaline'pI-l range 9.5 to 11.5, and although trisodium phosphate is the preferred phosphate, it will be'unclerstood that disodium phosphate may be employed, as well as pyro-.

phosphates, tetraphosphates, and various mixtures of these phosphates, the prime consideration being that the initial pH of the phosphate solutions used falls in the preferred pI-I limits. Although hydrogen peroxide as such is preferred as the addition peroxide 'inorder to obtain the desired pH, it will be understood that aqueous peroxide solutions maybe employed which were prepared by the use of alkali metal peroxides;

alkaline earth metal peroxides and other percompounds which can be converted in aqueous solution to produce active oxygen (or hydrogen peroxide).

After the alkaline peroxide solutioncontaining dump dewatering the web to a water content of less than about 250% on an oven dry pulp basis whereby the web is self-sustaining and thereafter applying to at least one face of the pulp web sufiicient of an aqueous alkaline bleaching solution to produce the desired degree of bleach in the web, said solution consisting of hydrogen peroxide and a soluble phosphate of an alkali metal as the essential ingredients, the latter compound producing a pH therein in the range PH 9.5 to 11.5.

2. The method of bleaching ground wood fibers which comprises felting the fibers into a web from a water suspension containing such fibers, dewatering the web to a water content of less than about 250% on an oven dried pulp basis whereby the web is self-sustaining and thereafter applying to at least one face of the pulp Web sufficient of an aqueous alkaline bleaching solution to produce the desired degree of bleaching in the web, said solution consisting of hydrogen peroxide and a soluble phosphate of an alkali metal as the essential ingredients, the latter compound producing a pH therein in the range pH 9.5 to 11.5 and subsequent to such application removing additional water from the web by evaporation.

ROBERT L. McEWEN.

FRED R. SHELDON.

6 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,409,799 Trostel Mar. 14, 1922 2,107,297 Kauffmarm Feb. 8, 1938 2,125,634 Heritage Aug. 2, 1938 2,150,926 Kauifmann Mar. 21, 1939 2,191,431 Kaufimann Feb. 20, 1940 2,435,566 Adams Feb. 10, 1948 FOREIGN PATENTS Number Country Date 21,631 Australia Mar. 4, 1935 of 1935 OTHER REFERENCES Reichert, Sodium Peroxide Bleaching of Mechanical Pulps. Paper Trade Journal, March 16, 1944, pages -52 (Tappi Section pages 89-96). (Pages -51 particularly pertinent.)

Kauffmann, Hydrogen Peroxide in the Pulp and Paper Industry. Paper Trade Journal, Nov. 8, 1945, pages 33 to 38 (Tappi Section pps.

MUUSH 

1. THE METHOD OF BLEACHING GROUND WOOD FIBERS WHICH COMPRISES FELTING THE FIBERS INTO A WEB FROM A WATER SUSPENSION CONTAINING SUCH FIBERS, DEWATERING THE WEB TO A WATER CONTENT OF LESS THAN ABOUT 250% ON AN OVEN DRY PULP BASIS WHEREBY THE WEB IS SELF-SUSTAINING AND THEREAFTER APPLYING TO AT LEAST ONE FACE OF THE PULP WEB SUFFICIENT OF AN AQUEOUS ALKALINE BLEACHING SOLUTION TO PRODUCE THE DESIRED DEGREE OF BLEACH IN THE WEB, SAID SOLUTION CONSISTING OF HYDROGEN PEROXIDE AND A SOLUBLE PHOSPHATE OF AN ALKALI METAL AS THE ESSENTIAL INGREDIENTS, THE LATTER COMPOUND PRODUCING A PH THEREIN IN THE RANGE PH 9.5 TO 11.5. 